1. Field of the Invention
The present invention relates to an electrophotographic machine, and, more particularly, to calibration of halftone screens in an electrophotographic machine.
2. Description of the Related Art
Color laser printers need regular calibration of halftone screens to ensure that printed colors, especially those colors made by mixing multiple color planes, are consistent and accurate. It is known to perform the calibration by measuring a reflection signal from test patches of different shade levels for each color plane. The test patches are sized to either allow multiple measurements on a patch or to sample over a wide area to minimize noise. The number of patches is limited by calibration time and patch size. Thus, the halftone response function is measured at a limited number of points, typically less than twelve. FIG. 1 shows a typical halftone response curve derived by interpolating between ten discrete measurement points.
The limited number of points sampled and the noise inherent in the measurement process limits the accuracy of the linearization process. That is, measuring discrete color patches and fitting a response function to the sample colors compromise halftone screen linearization. This misses important details in the shape of the halftone response curve, especially near zero coverage unless a large number of patches are sampled. An obvious alternative is to increase the number and size of the test patches, but this wastes more toner and increases the amount of time that the printer is unavailable for printing.
What is needed in the art is a method of performing halftone screen linearization that is accurate, quick, and does not use an excessive amount of toner.
The present invention provides a method of determining the shape of a halftone response curve by using only a single continuous gradient test patch.
The invention comprises, in one form thereof, a method of calibrating an electrophotographic machine having an image-bearing surface. The method includes the steps of depositing a toner patch on the image-bearing surface, the toner patch having a plurality of areas, the plurality of areas having different shade levels; emitting a light beam that extends over a region of multiple areas of the plurality of areas of the toner patch; measuring an amount of light that is reflected off of the multiple areas of the toner patch; repeating the emitting and measuring steps for each of a plurality of different regions of the toner patch; and adjusting at least one electrophotographic condition, the adjusting being dependent upon the measuring steps.
An advantage of the present invention is that a large number of test measurements can be performed on a single continuous gradient patch. Thus, the halftone response curve is determined more accurately than can be determined with discrete test patches.
Another advantage is that a large number of test points can be measured more quickly than by using discrete test patches.
Yet another advantage is a single continuous gradient test patch uses less toner than do multiple discrete test patches.